Paper manufacture is a capital intensive industry. Demands for increased productivity have led to papermaking machines which produce wider and wider webs. Currently, machines which yield a continuous web of paper in the range of 400 inches wide are known. Papermaking machines running at 6,000 feet per minute are now considered practical.
A papermaking machine can be divided into four sections: The forming section, where paper is formed from a dilute suspension of wood fibers in water and dewatered for example on a fourdrinier screen or wire. A pressing section where the newly formed mass of fibers is pressed to remove water until the remaining water content is thirty to seventy percent of the weight of the paper. A dryer section where the paper is dried to a moisture content generally in the neighborhood of five percent. And finally a winder where the paper is reeled up for transportation, storage, further processing or sale.
As papermaking speeds have increased, the size of the drying section has had to increase as well. Thus, the drying section of the papermachine represents a substantial capital cost especially as paper speeds have increased. The drying section also is the principal user of energy in the papermaking process. These attributes of the drying system have focused attention on improving the efficiency of the pressing section to decrease the moisture content from seventy percent to fifty percent or less. One method of achieving this is hot pressing in an extended nip press (ENP).
In an extended nip press an elongate concave shoe is pressed against a backing roll to define therebetween an extended pressing section for the passage therethrough of a paper web. A looped bearing blanket extends through the pressing section and slidably engages the concave surface defined by the shoe such that the web is carried by the blanket through the pressing section. A backing felt also extends through the pressing section and underlies the paper web.
The primary advantage of the extended nip press is the increased residence time of the web in the pressing section. More particularly, by heating the backing roll to a high temperature, water vapor generated within the extended pressing section further assists in pushing water remaining in the web in the liquid phase into the backing felt.
A problem that has been experienced with heated extended nip presses is the tendency for the pressed paper web to stick to the outer surface of the backing roll after the paper web has left the extended nip. In the past, granite rolls have been used in pressing sections of papermaking machines for the excellent release characteristics of their surfaces. The use of granite rolls presents several challenges in modern high temperature extended nip presses. The first is difficulty of supporting the somewhat brutal granite roll in contact with the extended nip, especially as the width of the paper web being manufactured becomes increasingly large. The second problem is the relatively low thermal conductivity of granite which limits the amount of heat which can be put into the paper web at high forming speeds. A third and not unimportant disadvantage of granite rolls is their high procurement costs. A fourth disadvantage is that heat can cause the granite roll to crack and fail.
Thus, because of the aforementioned problems of granite, metal backing rolls are utilized in high temperature extended nip presses. To overcome the problem of sticking, the upstream surface of the heated backing roll has been sprayed with an atomized layer of releasing agent. However, such releasing agents are not only relatively costly but present the possibility of deleteriously affecting the resulting pressed web. Experiments have been carried out with a steel backing roll with a chromium plated surface. However, such chromium plated surfaces have not been altogether successful in providing a uniform release of a pressed web.
What is needed is a backing roll with a surface which will readily release a paper web after hot pressing.